It is well known in the art to use steerable (or tracking) antennas to communicate with a relatively moving target over a wide scan angle. Especially in the aerospace industry, such steerable antennas preferably need to have high gain, low mass, and high reliability. The antennas used in wide scan applications typically include two rotation axes requiring two rotary joints, cable cassettes or other means of propagating the signal over each of the rotation axis. The elimination or the reduction of the number of RF (radio-frequency) rotary joints is highly desirable from a cost, signal loss and reliability perspective. Some solutions have been developed to eliminate rotary joints in wide angle steerable antennas but they are affected by the presence of a singularity which affects the ability to track a target when the beam becomes substantially aligned with one of the rotation axes. This singularity is referred to as the key-hole effect, because of the time required for the rotation around the axis presenting a singularity to keep up with the target rate of motion. Generally, for satellite based systems, this singularity is associated with the use of an azimuth rotation axis that points to the earth (sub-satellite point or nadir). For certain missions, this singularity has little impact on the overall system performance or complexity but in many cases, especially when a high gain is required, it can call for very high actuator speed in order to maintain an adequate antenna pointing as the targets gets close to a rotation axis. For a steerable antenna equipped with a nadir pointing azimuth rotation axis, this happens when the satellite ground track passes near the intended target. This can become a driver in the choice of the actuator and increase the complexity of the drive electronics system. Larger rotary actuators with more complex and costly drive electronics are then required. A solution having no rotary joints is illustrated in U.S. Pat. No. 6,747,604 issued on Jun. 8, 2004. This configuration suffers from a key-hole effect or singularity at nadir (pointing towards the Earth center for an antenna mounted on an Earth facing panel of an orbiting spacecraft) since one of the rotation axis is pointing towards nadir. The same key-hole effect also applies when a target on a GEO (Geostationary Earth Orbit) orbit is being tracked from a LEO/MEO (Low/Medium Earth Orbit) orbit.
Another solution having no key-hole or singularity at nadir but a RF rotary joint is shown in FIG. 1 (from US Patent Publication No. US 2014/01014125 A1 dated Apr. 17, 2014). This configuration has a rotary actuator R2 of a second axis A2 being mounted onto the rotary actuator R1 of the first axis A1, and still requires the use of either a cable cassette, slip ring, mobile harness or the like to transmit power and/or signal over the first rotation axis to/from the second rotary actuator, which approach incurs additional weight, mechanical/electrical complexity, limited pointing range and envelope, not saying additional overall cost.
Accordingly, there is a need for an improved steerable antenna configuration.